In recent years, in order to further improve the contrast of liquid crystal display devices, it has been considered introducing a front plate made of glass or acrylic resin on the viewing side of a conventional liquid crystal panel. Providing the front plate can reduce diffused reflection due to irregularities on a surface of a conventional polarizing plate protective film, and this ensures high-contrast and sharp expression of color. Further, it is also advantageous that the color does not change when the surface of a liquid crystal display device is pressed by a finger.
When such a front plate is pasted on a liquid crystal panel with a void therebetween, the void lowers the contrast because external light is multiply reflected on its interface. It is thus preferable to provide a sealing layer which fills the void for bringing the front plate and the liquid crystal panel into close contact. Among various methods of curing such a sealing layer such as thermal curing and sticking, the most suitable method is ultraviolet curing using ultraviolet curable resin as the sealing layer.
For example, for bringing a liquid crystal panel and a front plate into close contact via a photocurable resin, patent literature 1 discloses a technique of avoiding deterioration of display quality without damaging a liquid crystal panel by irradiating the photocurable resin only with light at a wavelength of 340 nm or more and blocking light at a wavelength less than 340 nm, which has a negative influence on liquid crystal molecules, adhesives and the like.
On the other hand, glass or acrylic front plates are not used alone, but are generally provided with a resin film or the like having a hard coat layer for preventing scratch or reflection. Thus, taking productivity into consideration, a preferable method is providing a sealing layer containing an ultraviolet curing sticking agent, a front plate composed of glass or an acrylic resin, an adhesive layer, and a resin film having a hard coat layer in this order on the surface on the viewing side of a liquid crystal panel which has polarizing plates on both faces of the liquid crystal cell, followed by irradiation with ultraviolet ray from the hard coat layer side so as to cure the sealing layer.
There are various kinds of such resin films, and polycarbonate films (PC film), polyethylene terephthalate films (PET films), cellulose acylate films and the like are in practical use. However, polycarbonate films (PC films) and polyethylene terephthalate films (PET film) which are conventionally used for front plates have large phase differences, and thus these films may cause moire when used on the surface of the front plates resulted in unfavorable display quality.
In contrast, triacetylcellulose films (TAC films) having hard coat layer, which have conventionally been used for polarizing plates and liquid crystal display devices, are advantageous as a hard coat film to be pasted on a front plate, because TAC films do not cause moire by their low phase difference and thus have high transparency and good processability. However, when the sealing layer is cured by ultraviolet irradiation from the hard coat layer side, an ultraviolet absorber contained in the TAC film absorbs the ultraviolet ray for curing the sealing layer. The amount of the ultraviolet ray is thus decreased before reaching the sealing layer, and the sealing layer cannot be cured sufficiently. Therefore, decrease in contrast is caused by delamination or distortion between the liquid crystal panel and front plate.
If the amount of the ultraviolet ray is increased so that the sealing layer can receive a sufficient amount of light for curing, the TAC film with a hard coat layer is subjected to heat and the ultraviolet ray itself. It causes wrinkles or distortion between the hard coat layer and the base film and delamination of the hard coat layer, which have been another problem.